A “lead frame” is a device commonly used to attach semiconductor dies to a circuit board. An example follows of how a typical lead frame may be used to attach a semiconductor die to a circuit board. The semiconductor die is attached to an inner portion of the lead frame, and wire bonds are run from the semiconductor die to conductive leads integral to the lead frame. The lead frame has a body that allows the lead frame to be picked up and moved during procedures such as bonding of the wire bonds from the semiconductor die to the conductive leads. The body also surrounds the conductive leads, which are initially flat and supported by a tie bar, which is connected to the body.
Once the semiconductor die is connected to the conductive leads, molding is performed so that mold compound fills the inner and other portions of the lead frame. The conductive leads are typically made of copper (Cu), although other materials may be used. Cu tends not to bond well with materials used during soldering of the conductive leads to a circuit board. Consequently, the copper is generally plated with another metal, such as tin (Sn), which does bond well with soldering materials.
A “trim and form” process is performed, during which the tie bar is removed, as is the body of the lead frame. The trim and form process additionally forms the conductive leads into particular shapes suitable for soldering to pads of a circuit board.
While the lead frame and its processing have been used for many years, certain types of conductive leads can have growth formation. For instance, Sn-plated Cu has growths of Sn, called “whiskers,” that extend from the surface of the conductive lead. These whiskers have been documented since about 1946, and failures attributed to the whiskers have been documented as early as 1954.
One solution that minimizes or prevents whisker formation is plating the Cu with a lead (Pb)—Sn layer. Recently, however, legislation in Europe requires that Pb be eliminated from electronic devices.
A need therefore exists for Pb-free plated conductive leads, devices made therefrom and methods for forming the same that also have limited or no growth formations.